The present invention relates to a variable valve timing control apparatus of an internal combustion engine, which variably controls open and closing timing of an intake valve and/or an exhaust valve of the engine according to an engine operating condition.
In recent years, there have been proposed and developed various variable valve timing control apparatuses. One such variable valve timing control apparatus has been disclosed in Japanese Patent Provisional Publication No. 2004-11537 (hereinafter is referred to as “JP2004-11537”) corresponding to U.S. Pat. No. 6,805,081 (B2), applied for by this applicant.
The variable valve timing control apparatus disclosed in JP2004-11537 includes a timing sprocket to which torque is transferred from a crankshaft of an engine, a camshaft rotatably supported within a predetermined angular range with respect to the timing sprocket, a sleeve fixedly connected to the camshaft, and a rotational phase control mechanism (or a relative angular phase control mechanism) provided between the timing sprocket and the sleeve so as to control a rotational phase of the camshaft relative to the timing sprocket in accordance with an engine operation condition.
The rotational phase control mechanism includes a radial guide groove formed in the timing sprocket, a spiral guide (a concentric-spiral guide groove) formed on a surface of a spiral guide disk, a link member having two ends: an inner end pivotally fixed to the sleeve and an outer end slidably supported in the radial guide groove so that the outer end can slide in a radial direction along the radial guide groove, an engagement portion provided at the outer end of the link member, the engagement portion has a spherical portion (a semi-spherical protrusion) engaged with the spiral guide, and a hysteresis brake applying a braking force to the spiral guide disk according to an engine operating condition.
The hysteresis brake has at the front end side of the sleeve a coil yoke, and an electromagnetic coil circumferentially surrounded with the coil yoke. The coil yoke has at a rear side thereof a pair of circumferentially-opposed cylindrical surfaces with a cylindrical air gap left between the opposed surfaces. The coil yoke further has a plurality of pole teeth on the opposed surfaces respectively. Furthermore, a bottomed and cylindrical-shaped hysteresis member, which has a hysteresis characteristic of magnetic flux, is arranged in the air gap between the opposed surfaces (in the air gap between the opposed pole teeth). The hysteresis member is movable relative to the opposed pole teeth.
When the electromagnetic coil is energized, a magnetic field is induced between the opposed pole teeth across the hysteresis member, and thereby an electromagnetic brake acts on the spiral guide disk via the hysteresis member. By way of this action (braking on the spiral guide disk), the engagement portion is guided along the spiral guide while the engagement portion moves in the radial direction along the radial guide groove. Thus, the sleeve (also the camshaft) can be rotated relative to the timing sprocket within a predetermined angular range.